ABSENCE OF CIRCADIAN-RHYTHM IN THE DISPOSITION OF MELATONIN IN THE COW

Five adult pasture-bred Prim Holstein cows were used to document the t ime invariance of melatonin disposition in the cow. Melatonin was admi nistered by an intravenous route as a bolus (5 mu g/kg of body weight) at either 1430 or 2230. Data were analysed according to a three-compa rtment open model with melatonin elimination from the central compartm ent. The mean (+/-SD) kinetic parameters describing melatonin disposit ion, during day and night, respectively, were plasma clearances: 0.026 7 +/- 0.0084 1/kg per min and 0.0229 +/- 0.0031 1/kg per min; steady s tate volume of distribution: 0.826 +/- 0.229 1/kg and 0.780 +/- 0.193 1/kg; terminal half lives: 58.9 +/- 23.7 min and 64.1 +/- 33.7 min; me an residence times: 32.0 +/- 8.8 min and 33.9 +/- 6.2 min. No statisti cal difference was found between diurnal and nocturnal values for thes e parameters. These results support the concept of time invariance for melatonin kinetics in cattle and validate the use of diurnal experime nts for the study of melatonin kinetics.